I believe heartfelt letters are very powerful. There are a few rules of letter writing that help. Talk about yourself, not her. Avoid saying things like "you did....." or "you said...", that comes off as accusatory. Stick to "I felt.....", and "I was....". And it is powerful to talk about your feelings, not what you think. You can say, "I felt angry, or hurt, or any adverb. But don't say, "I felt like you did...." which is not a feeling and is accusatory. Anytime you say "I felt like...." you are really saying "I think....", and thinking and feeling are not the same. When you talk about what you think there is room for argument. When you talk about how you feel, who can argue with that? Your feelings are real and they are yours. Yes, write a letter. She can read it now and perhaps not be moved by it. But if she keeps it, she may read it again a few months from now and be touched. Words have power.